1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to a liquid crystal display device implementing in-plane switching (IPS) where an electric field to be applied to liquid crystal is generated in a plane parallel to a substrate.
2. Discussion of the Related Art
A typical liquid crystal display (LCD) device uses optical anisotropy and polarization properties of liquid crystal molecules. The liquid crystal molecules have a definite orientational order in alignment resulting from their thin and long shapes. The alignment orientation of the liquid crystal molecules can be controlled by supplying an electric field to the liquid crystal molecules. In other words, as the alignment direction of the electric field is changed, the alignment of the liquid crystal molecules also changes. Because incident light is refracted to the orientation of the liquid crystal molecules due to the optical anisotropy of the aligned liquid crystal molecules, image data is displayed.
A liquid crystal is classified into a positive liquid crystal and a negative liquid crystal, in view of electrical property. The positive liquid crystal has a positive dielectric anisotropy such that long axes of liquid crystal molecules are aligned parallel to an electric field. Whereas, the negative liquid crystal has a negative dielectric anisotropy such that long axes of liquid crystal molecules are aligned perpendicular to an electric field.
By now, an active matrix LCD that the thin film transistors and the pixel electrodes are arranged in the form of a matrix is most attention-getting due to its high resolution and superiority in displaying moving video data.
FIG. 1 is a cross-sectional view illustrating a typical twisted nematic (TN) LCD panel. As shown in FIG. 1, the TN-LCD panel has lower and upper substrates 2 and 4 and an interposed liquid crystal layer 10. The lower substrate 2 includes a first transparent substrate 1a and a thin film transistor (“TFT”) “S”. The TFT “S” is used as a switching element to change orientation of the liquid crystal molecules. The lower substrate 2 further includes a pixel electrode 15 that applies an electric field to the liquid crystal layer 10 in accordance with signals applied by the TFT “S”. The upper substrate 4 has a second transparent substrate 1b, a color filter 8 on the second transparent substrate 4, and a common electrode 14 on the color filter 8. The color filter 8 implements color for the LCD panel. The common electrode 14 serves as another electrode for applying a voltage to the liquid crystal layer 10. The pixel electrode 15 is arranged over a pixel portion “P,” i.e., a display area. Further, to prevent leakage of the liquid crystal layer 10 between the lower and upper substrates 2 and 4, those substrates are sealed by a sealant 6.
As described above, because the pixel and common electrodes 15 and 14 of the conventional TN-LCD panel are positioned on the lower and upper substrates 2 and 4, respectively, the electric field induced therebetween is perpendicular to the lower and upper substrates 1a and 1b. The above-mentioned liquid crystal display device has advantages of high transmittance and aperture ratio, and further, since the common electrode on the upper substrate serves as an electrical ground, the liquid crystal is protected from a static electricity.
FIGS. 2A and 2B show different alignments of the positive TN liquid crystal molecules 10, respectively, without and with an electric field (off and on states). In FIG. 2A, various arrows show the gradual rotating of the liquid crystal molecules 10 with polar angles 0 to 90 degrees, which are measured on a plane parallel to the lower and upper substrate 2 and 4. At the same time, the liquid crystal molecules 10 are gradually rotated to 90 degrees from the lower substrate 2 to the upper substrate 4. That is to say, the long axes of the liquid crystal molecules 10 gradually rotate along a helical axis (not shown) that is perpendicular to the lower and upper substrates 2 and 4. First and second polarizers 18 and 30 are positioned on the exterior surfaces of the lower and upper substrate 2 and 4, respectively. At this point, the broken lines on the first and second polarizers 18 and 30 correspond to first and second transmittance axis of the first and second polarizers 18 and 30, respectively. After rays of light travel through a TN liquid crystal panel in the off state, as discussed above, they are linearly polarized and rotated 90 degrees.
As shown in FIG. 2B, when there is an electric field “E” applied to the positive TN liquid crystal molecules 10, the liquid crystal molecules are aligned perpendicular to the upper and lower substrates 4 and 2. That is to say, with the electric field E applied across the liquid crystal molecules 10, the liquid crystal molecules 10 rotate to be parallel to the electric field “E”. In this case, the rotation of the linearly polarized light does not take place. Therefore, light is blocked by the second polarizers 30 after it travels through the first polarizer 18.
However, the above-mentioned operation mode of the TN-LCD panel has a disadvantage of a narrow viewing angle. That is to say, the TN liquid crystal molecules rotate with polar angles 0 to 90 degrees, which are too wide. Because of the large rotating angle, contrast ratio and brightness of the TN-LCD panel fluctuate rapidly with respect to the viewing angles.
To overcome the above-mentioned problem, an in-plane switching (IPS) LCD panel was developed. The IPS-LCD panel implements a parallel electric field that is parallel to the substrates, which is different from the TN or STN (super twisted nematic) LCD panel. A detailed explanation about operation modes of a typical IPS-LCD panel will be provided with reference to FIGS. 3, 4A, and 4B.
As shown in FIG. 3, first and second substrates 1a and 1b are spaced apart from each other, and a liquid crystal “LC” is interposed therebetween. The first and second substrates 1a and 1b are called an array substrate and a color filter substrate, respectively. Pixel and common electrodes 15 and 14 are disposed on the first substrate 1a. The pixel and common electrodes 15 and 14 are parallel with and spaced apart from each other. On a surface of the second substrate 1b, a color filter 25 is disposed opposing the first substrate 1a. The pixel and common electrodes 15 and 14 apply an electric field “E” to the liquid crystal “LC”. The liquid crystal “LC” has a negative dielectric anisotropy, and thus it is aligned parallel to the electric field “E”.
FIGS. 4A and 4B conceptually illustrate operation modes for a typical IPS-LCD device. In an off state, the long axes of the LC molecules “LC” maintain a definite angle with respect to a line that is perpendicular to the pixel and common electrodes 15 and 14. The pixel and common electrode 15 and 14 are parallel with each other. Herein, the angle difference is 45 degrees, for example.
In an on state, an in-plane electric field “E”, which is parallel with the surface of the first substrate 1a, is generated between the pixel and common electrodes 15 and 14. The reason is that the pixel electrode 15 and common electrode 14 are formed together on the first substrate 1a. Then, the LC molecules “LC” are twisted such that the long axes thereof coincide with the electric field direction. Thereby, the LC molecules “LC” are aligned such that the long axes thereof are perpendicular to the pixel and common electrodes 15 and 14.
In the above-mentioned IPS-LCD panel, there is no transparent electrode on the color filter, and the liquid crystal used in the IPS-LCD panel includes a negative dielectric anisotropy.
FIGS. 5A and 5B are conceptual plane views illustrating alignment of the liquid crystal molecules of the above-mentioned IPS-LCD panel, respectively, in off and on states. As shown in FIG. 5A, each liquid crystal molecule 10 is aligned in a proper direction by a pair of alignment layers (not shown), which are formed on opposing surfaces of the first and second substrate 1a and 1b. As shown in FIG. 5B, the electric field “E” is applied between the pixel and common electrodes 15 and 14 such that each molecule 10 is aligned in accordance with the electric field “E”. That is to say, each liquid crystal molecule 10 rotates to a definite angle in accordance with the electric field “E”.
Compared with the TN-LCD device of FIG. 1, the IPS-LCD device has a wider viewing angle owing to a smaller rotating angle of the liquid crystal molecules.
The IPS-LCD device has the advantage of a wide viewing angle. Namely, when a user looks at the IPS-LCD device in a top view, the wide viewing angle of about 70 degrees is achieved in up, down, right and left directions.
By the above-mentioned operation modes and with additional elements such as polarizers and alignment layers, the IPS-LCD device displays images. The IPS-LCD device has a wide viewing angle, low color dispersion qualities, and the fabricating processes thereof are simpler among those of various LCD devices.
However, because the pixel and common electrodes are disposed on the same plane on the lower substrate, the transmittance and aperture ratio are low. In addition, a response time according to a driving voltage should be improved, and a color's dependence on the viewing angle should be decreased.
FIG. 6 is a graph of the CIE (Commission Internationale de l'Eclairage) color coordinates and shows the color dispersion property of the conventional IPS-LCD device. The horseshoe-shaped area is the distribution range of the wavelength of visible light. The results are measured using point (0.313, 0.329) in CIE coordinate as a standard white light source and with various viewing angles of right, left, up and down, and 45 and 135 degrees. Obviously, the range of the color dispersion is so long, which means that the white light emitted from the conventional IPS-LCD device is dispersed largely according to the viewing angle. This results from the fact that the operation mode of the IPS-LCD device is controlled by birefringence. S. Endow et al. indicated the above-mentioned problem in their paper “Advanced 18.1-inch Diagonal Super-TFT-LCDs with Mega Wide Viewing Angle and Fast Response Speed of 20 ms: IDW 99′ 187 page”.
FIG. 7 is a graph illustrating transmittance with respect to viewing angles for first to eighth gray levels (gray scale) of a conventional IPS-LCD device. Except for the first gray level, “level 1,” each gray level has the highest transmittance at a viewing angle of 0 degree. The first gray level, “level 1,” has gray inversion regions. When the viewing angle is beyond 60 degrees, the first gray level, “level 1,” has the higher transmittance than the fourth gray level, “level 4.” The first gray level, “level 1,” should implement a black state of the LCD panel. However, gray inversion occurs at viewing angles larger than 60 degrees, such that a white state, but not a black state, is produced at the larger viewing angles. The above-mentioned gray inversion results from a birefringence dependence of the IPS-LCD device and causes a poor display quality of the IPS-LCD device.
FIG. 8 shows an example of the IPS-LCD device according to the related art. As shown in FIG. 8, zigzag-shaped pixel electrodes 35 and zigzag-shaped common electrodes 34 are alternately arranged such that first and second electric fields 46a and 46b are alternately induced along the zigzag-shaped electrodes. The first and second electric fields 46a and 46b have different directions. Therefore, a multi-domain is achieved owing to the first and second electric fields 46a and 46b. An alignment layer (not shown) is also used for a first state alignment of liquid crystal molecules (reference 10 of FIG. 3). The alignment layer (not shown) beneficially has one rubbing direction 44.
The above-mentioned zigzag-shaped common and pixel electrodes 34 and 35 minimize the color shift. However, between bending portions “D” of the common and pixel electrodes 34 and 35, an electric field is induced perpendicular to the rubbing direction 44. That is to say, long axes of the liquid crystal molecules are perpendicular to the electric field induced between the bending portions “D.” Then, the liquid crystal molecules cannot rotate, but keep the first state alignment such that an abnormal alignment is present at each boundary portion “C” between the different domains.
The abnormal alignment at the boundary portion “C” causes a light leak such that white lines are shown on a display area, the pixel region “P” shown in FIG. 1, of the LCD device. The above-mentioned white lines are called a disclination. A black matrix may be expanded to the pixel regions to cover the disclination. However, the expanded black matrix causes a low aperture ratio.
Now, with reference to FIGS. 9A and 9B, effect of the multi-domain is explained in detail. A liquid crystal layer generally has a birefringence, because each liquid crystal molecule has a long and thin shape. The birefringence changes with respect to a viewing angle. FIG. 9A is a cross-sectional view illustrating a single-domain for a liquid crystal molecule 10 between upper and lower polarizers 30 and 18. At this point, the birefringence of the liquid crystal molecule 10 involves different values for the first, second, and third position “a”, “b”, and “c”, which involve different viewing angles. Therefore, the birefringence of the liquid crystal molecule 10 cannot be zero with respect to viewing angles. If the birefringence of the liquid crystal layer is not zero, the perfect black state cannot be achieved between the upper and lower polarizers 30 and 18.
To overcome the above-mentioned problem, the multi-domain shown in FIG. 9B is adopted for a LCD device. As shown, there are first and second liquid crystal molecules 10a and 10b arranged opposite to each other. The birefringence of the first liquid crystal molecule 10a involves different values for the first, second, and third position “a1”, “b1”, and “c1.” Whereas, the birefringence of the second liquid crystal molecule 10b involves different values for the fourth, fifth, and sixth position “a2”, “b2”, and “c2.” The first and fourth positions “a1” and “a2” involve the same viewing angle. Because the first and second liquid crystal molecules 10a and 10b are symmetrically opposed with each other, birefringence of the first liquid crystal molecule 10a at the first position “a1” is compensated by that of the second liquid crystal molecule 10b at the fourth position “a2.” That is to say, each birefringence of the first liquid crystal molecule 10a is compensated by corresponding birefringence of the second liquid crystal molecule 10b. In other words, sum of the birefringence between the first and second liquid crystal molecules 10a and 10b is about zero. Accordingly, the multi-domain shown in FIG. 9B improves the display quality of the LCD device.